The present invention relates to a sintered material and a composite sintered contact component. More particularly, the present invention relates to a Cuxe2x80x94Al-based sintered material and a composite sintered contact component manufactured by use of the Cuxe2x80x94Al-based sintered material. The Cuxe2x80x94Al-based sintered material is obtained by increasing the sinterability of Al-bronze alloys widely used as copper alloys having excellent hardness, wear resistance, high-temperature oxidation resistance and corrosion resistance, and therefore is suited for use in manufacture of products with good dimensional accuracy.
Al-bronze alloys are widely used as copper alloys having high hardness, wear resistance, high-temperature oxidation resistance and corrosion resistance. However, when producing an Al-bronze alloy component from sintered material, abnormal expansion occurs during a sintering process, making it difficult to compress the material. For this reason, Al-bronze cast alloys and particularly, Cuxe2x80x94Alxe2x80x94Fexe2x80x94Nixe2x80x94Mn alloys are most commonly used and these alloys are stipulated as xe2x80x9cAlBC1-4xe2x80x9d by Japan Industrial Standard.
Related prior art is disclosed in Japanese Patent Publication (KOKAI) Gazette Nos. 56-152901 (1981) and 56-152902 (1981) according to which, 0.1 to 10 wt % Ti or 0.05 to 1.0 wt % P is added for the purpose of encouragement of sintering, thereby achieving Cuxe2x80x94Al-based sintered materials excellent in strength and toughness.
For example, bronze and lead-bronze based materials such as Cuxe2x80x94Snxe2x80x94Pb are often used as copper-based sintered bearing materials, and double-layered sintered contact components in which one of such sintered materials is integral with an iron backing are well known. Such contact components are commonly used for the rollers incorporated in the base carrier of construction machines.
Also, steel bushings, to which carburization or induction hardening focused upon wear resistance has been applied, are commonly used in grease-lubricated circumstances as bearings (e.g., implement bushings for construction machines) used under higher bearing pressure and lower speed conditions. In particularly these implements, lubrication is getting worse under high bearing pressure, making an unpleasant abnormal noise in operation. Attempts to prevent abnormal noise have been made by use of high strength brass bushings or bushings made by further applying lubricant coating treatment to the above-described steel bushings.
An Al-bronze-based, double-layered, sintered contact component used in a high bearing pressure condition is disclosed in Japanese Patent Publication (KOKAI) Gazette No. 5-156388 (1993). According to this publication, an Al-bronze-based sintered alloy powder sheet, in which 3 to 8 wt % graphite (as a solid lubricating element), 5 to 13 wt % Al, 3 to 6 wt % Fe and 0.1 to 1.5 wt % Ti are dispersed, is bonded to a steel plate with a phosphor-bronze bonded layer therebetween, and at that time, pressure is applied during sintering at 800 to 950 degrees centigrade to provide high density for the Al-bronze-based sintered layer while firm bonding is ensured. In the sintered layer of the double-layered sintered contact component disclosed in the above publication, Ti is added in the form of hydrogenated Ti (TiH), while the Al2O3 coating of the Al powdery layer is reduced by hydrogen generated during sintering to increase sinterability. The sintered layer contains 18 to 25% by volume of voids and these voids are impregnated with a lubricant, thereby forming a contact component.
Al-bronze alloys widely used as high-strength, wear resistant copper-based alloys, however, have revealed the disadvantages that Al2O3 suspends during dissolution, causing poor fluidity and that they cause violent gas absorption, leading to a high coefficient of coagulation/contraction. For this reason, it is difficult to form sound cast products from Al-bronze alloys. Accordingly, a need exists for easy development of Al-bronze sintered alloys. However, as disclosed by Mitani et al. (xe2x80x9cRevised and Enlarged Edition of Powder Metallurgyxe2x80x9d pp. 79-82; pp. 258-260 issued by Corona Publishing Co., Ltd. (Sep. 10, 1985)), sound products having good compactness and dimensional accuracy cannot be produced from Cuxe2x80x94Al-based sintered materials since considerable expanding phenomenon emerges during sintering.
Hashimoto et. al. have reported an Al adding process in which compaction is carried out by sintering a Cuxe2x80x94Al-based alloy powder containing 6.54 wt % Al or 9.92 wt % Al at a high temperature of 1,000 degrees centigrade (xe2x80x9cPowder and Powder Metallurgyxe2x80x9d, Vol. 29, No. 6, p. 211 (1982)). This process also suffers from the problem that an extremely strong degree of springback occurs when a compact particularly formed from a mixture of electrolytic Cu and alloy powder is taken out of a die with the result that the green compact is substantially broken.
In addition, as pointed out in the above report written by Mitani et. al., the techniques disclosed in the aforesaid Japanese Publication Nos. 56-152901, 56-152902, which use a mixed powder or alloy powder containing, as a master alloy powder, a sintered material having a high concentration of Al (6 to 9 wt %), is directed to avoiding eutectic reaction at a temperature of 548 degrees of centigrade shown in the Cuxe2x80x94Al phase diagram, but have revealed such a problem that tendency for the springback of the compact is high and the alloy powder is hard, which make it difficult to increase compact density. Especially, a higher degree of springback leads to damage to the compact when it is removed from the die, resulting in a considerable increase in the percentage of defective products.
It is conceivable that springback may be reduced by sintering a compact in which the sintered material structure is adjusted to consist of an alpha single phase region by use of the above-described Cuxe2x80x94Al alloy powder and by utilizing the sinter promoting action of Ti and P which occurs during sintering. However, where a compact formed from Al or an Al alloy powder is sintered, the eutectic reaction is involved in sintering so that the sinter promoting action of Ti and P cannot be utilized without arrangement and as a result, there arises a need for an addition of other elements as a third element and its effect has to be studied.
This is apparent from the fact that as disclosed in Japanese Patent Publication No. 5-156388, a Cuxe2x80x94Al-based sintered contact material, in which 0.1 to 1.5 wt % TiH is added to a powder blend containing a pure Al powder to improve sinterability, has 18 to 25% by volume of voids in heat-sintering at a pressure of 5 kg/cm2 or less so that sufficient compactness cannot be achieved. Of course, the compactness of the sintered body can be achieved by applying increased pressure like the hot-press, but the application of increased pressure is disadvantageous in view of productivity as well as cost performance and, moreover, causes difficulty in producing sintered products of more intricate shape.
The double-layered sintered contact component of Japanese Patent Publication No. 5-156388 in which an Al bronze based sintered contact material containing 3 to 8 wt % graphite is integrally bonded to a metal backing with a phosphor bronze layer therebetween cannot avoid the increased cost of the sintering and/or sinter bonding process during which pressure is applied to cope with the above-described emergence of abnormal expansion during sintering. In addition, sinterability further decreases in the case of sintered metal bodies containing large amounts of solid lubricant such as graphite, and it is obvious that if high density and high hardness cannot be achieved in the sintered material, wear rapidly occurs in applications to implement bushings for construction machines which are subjected to use under an extremely high bearing pressure condition or a condition susceptible to a shortage of lubricant.
The Cuxe2x80x94Snxe2x80x94Pb lead bronze based sintered contact materials, which are commonly used for manufacturing rollers of the base carrier of construction machines, contain large amount of Pb. Therefore, a need exists for development of alternative materials that can be used in place of Pb in order to cope with the environmental problems.
Where the above-described Al bronze based sintered contact material containing 3 to 8 wt % of graphite is used as an alternative material for Pb, another problem arises in which coefficient of friction increases because of graphite dispersion, increasing the likelihood of heat development when the resultant component is in sliding operation.
As an attempt to solve the above problem, high-strength brass alloys attract attention because they are unlikely to seize even when the lubricant runs out. They are, in fact, used in part of implement bushings for construction machines, but have not reached a point where satisfactory functions can be achieved.
In addition, the sintering of high-strength brass alloys with intention of improving the sliding function of the resultant component, has revealed the following problem. It is very difficult to form a high-density sintered material from high-strength brass alloys containing large amounts of Zn having extremely high vapor pressure. Since the concentration of Zn in the sintered material is likely to fluctuate and a slight fluctuation of Zn concentration causes a significant fluctuation in the (alpha+beta) dual phase structure (base structure) of high-strength brass. As a result, the beta phase cannot be controlled, the beta phase highly affecting wear resistance and slidability which provides insusceptibility to seizure at the time of a shortage of the lubricant.
The present invention is directed to overcoming the foregoing problems and a prime object of the invention is therefore to provide a sintered material with high dimensional accuracy by increasing the sinterability of Cuxe2x80x94Al-based sintered material and to provide sintered contact components as well as composite sintered contact components, these components being excellent in strength, wear resistance, seizure resistance and corrosion resistance and formed from the above sintered material.
Another object of the invention is to provide a sintered material having a sintered structure in which a beta phase having a harder phase in the Cuxe2x80x94Al phase diagram has emerged therein and in which intermetallic compounds are dispersed within an (alpha+beta) dual phase, the beta phase and the bases of the (alpha+beta) dual phase and beta phases, with intention of increasing the wear resistance of a bearing used under high bearing pressure and preventing abnormal noises, and to provide composite sintered contact components produced by sinter-bonding the above sintered material to a metal backing in an integral fashion.
The term, xe2x80x9cbeta phasexe2x80x9d appearing in this specification is defined as a beta phase state at sintering temperature. It is well known that, as seen from Cuxe2x80x94Zn and Cuxe2x80x94Al phase diagrams, most of the constituents of the beta phase are martensite-transformed into a betaxe2x80x2 phase when the sintered material has been cooled down to room temperature after sintering. Therefore, the meaning of the term xe2x80x9cbeta phasexe2x80x9d herein includes the state of the betaxe2x80x2 phase.
The above objects can be achieved by a sintered material according to a first invention, which is a Cuxe2x80x94Al-based sintered material containing at least 1 to 12 wt % Sn and 2 to 14 wt % Al.
In the material of the first invention, the preferable relationship between the percentage of Al by weight and the percentage of Sn by weight is represented by:
18.5xe2x89xa62.5xc3x97(Al wt %)+(Sn wt %). 
Preferably, the sintered material of the first invention contains one or more of Ti within the range of 0.3 to 5 wt % and Si within the range of 0.5 to 3 wt %. Preferably, the sintered material of the first invention has a structure in which a beta phase is present at least within a sintered structure and intermetallic compounds are dispersed within an (alpha+beta) dual phase, the beta phase and/or the bases of the (alpha+beta) dual phase and the beta phase. In addition, the sintered material preferably contains elements such as Mn, Ni and Fe in an amount of 5 wt % or less, these elements stabilizing the beta phase, retarding the eutectoid transformation of beta=alpha+gamma, and imparting hardness. The sintered material preferably contains 2 wt % or less of P in the form of phosphor-iron alloy powder, P functioning to increase reducibility in sintering. Preferably, the sintered material contains one or more alloy elements selected from the group consisting of P, Zn, Fe, Ni, Co, Mn, Be, Pb, Mo, W, Mg and Ag and/or one or more dispersion elements such as WC, graphite and ceramics.
The sintered material of the invention is suited for use in sliding parts.
The invention utilizes Sn and/or Si as an alloy element which prevents the expansion of Cuxe2x80x94Al-based sintered material or contracts Cuxe2x80x94Al-based sintered material, even when Al or an Al alloy powder, which has extremely low tendency for springback after compaction, is used as an Al source. Further, the use of Sn and/or Si in combination with other alloy elements such as Ti, Ni, Mn and phosphor iron makes it possible to produce a Cuxe2x80x94Al-based sintered material having excellent sinterability. The details will be described below.
(a) Springback at the time of compaction was studied, using master alloy powders having an alpha or beta single phase and Al powders as an Al source. It was found from the study that where a beta single phase master alloy (13.9 wt % Al) was employed and a Cuxe2x80x94Al sintered material (mixed powder) containing 8 wt % Al was compacted at a pressure of 4 ton/cm2, springback was 0.57% and there was the danger of damage to the compact during removal from the die. In contrast with this, where an Al powder was utilized, there was no fear of breakage of the compact due to springback and the addition of an Al powder was found to be favorable upon condition that the abnormal expansion after sintering can be restricted. Therefore, Cuxe2x80x94Al-based sintered materials containing Al or an Al alloy powder as an Al source have been developed in the invention.
Regarding the sinterability of compacts, the features as shown in FIG. 1 were found:
(b) The sinterability of sintered materials having an alpha single phase composition was checked at a temperature of 1,000 degrees centigrade using alpha and beta phase master alloys. It was found from the test that where an alpha single phase master alloy was used, contraction was admitted although its degree was small, whereas where a beta single phase master alloy was used, noticeable expansion was observed.
(c) Where a Cuxe2x80x94Al master alloy having a beta single phase and containing 14 wt % Al was used and an Alxe2x80x94Cuxe2x80x94TiH sintered material having an (alpha+beta) dual phase sintered structure and containing 8 wt % Al and 1 wt % TiH was tested, the sintered material exhibited higher expandability than that of the above material so that sintering of (alpha+beta) dual phase alloys was found to be difficult.
(d) The sintering behavior of sintered materials containing Al powder
It has been found that, regarding Cuxe2x80x94Al binary sintered materials, expansion proceeds substantially in proportion to the concentration of Al at a sintering temperature of 1,000 degrees centigrade or less, but when the temperature of sintering is 1,020 degrees centigrade which is close to the eutectic temperature (1,037 degrees centigrade) of Cuxe2x80x94Al binary alloys, alloys having a structure more similar to the eutectic composition (8.5 wt % Al) have better sinterability.
It will be understood from the above findings that while the expansion during sintering is difficult to be restricted where a Cuxe2x80x94Al alloy powder having high Al concentration is used as an Al source, the sinterability of materials having a structure similar to the eutectic composition can be promoted at sintering temperatures close to the eutectic temperature, although a transitional liquid phase is generated. Accordingly, in the invention, sinterability is increased by addition of alloy elements such as Sn and Si which generate a stable liquid phase on the lower temperature side.
The effect when Ti was added to Cuxe2x80x94Al up to 3 wt % was checked. It was observed that although Ti did not promote sinterability nor contribute to the compaction of the sintered body at sintering temperatures of 1,000 degrees centigrade or less, Ti could achieve compaction at a sintering temperature of 1,020 degrees centigrade which was close to the eutectic temperature (1,038 degrees centigrade) of Cuxe2x80x94Al alloys. As discussed earlier, this is due to a decrease in the eutectic temperature caused by the addition of Ti, and the effect of the addition of Ti alone is limited to the particular temperature range, that is, temperatures just below the eutectic temperature of Al. Accordingly, it has been found that the sinterability of Cuxe2x80x94Al alloys can not be sufficiently improved by an addition of Ti alone.
It has been found that a satisfactory sinter promoting effect cannot be obtained by an addition of TiH which actively reduces Al oxide films, but where a liquid phase is sufficiently involved, a satisfactory sinter promoting effect can be achieved although this effect is limited to the particular temperature range (temperatures immediately below the eutectic temperature of Al). Accordingly, the inventors have found from the following knowledge that Sn can be effectively used as the third alloy element for promoting the sinterability of Cuxe2x80x94Al alloys.
(a) Even if the oxide films formed on the Al particles function to impede sintering, diffusivity can be extremely increased, promoting sinterability and high compacting (contraction) action can be allowed to emerge, by controlling, with the third element, the sintering condition so as to promote liquid phase sintering.
(b) The third element markedly reduces the melting point of Cu, and it is preferable that the dual phase region where (alpha+liquid phases) coexist be wide and the solid soluble region for the alpha phase be wide.
(c) The third element is unlikely to form intermetallic compounds, reacting with the coexisting Al element.
(d) If the third element forms intermetallic compounds, reacting with the coexisting Al element, the melting point of the intermetallic compounds is lower than the sintering temperature.
The sinter promoting effect is admitted in sintering at 1,000 degrees centigrade with about no less than 5 wt % Sn and in sintering at 900 degrees centigrade with about 11 wt % Sn. The sinter promoting effect of Sn is remarkably enhanced by an addition of Ti. For instance, the sinter-contraction of Cu-10Al-3S-1Ti is admitted at 1,000 degrees centigrade and remarkably enhanced at 960 degrees centigrade with an addition of 3 wt % Ti.
The reason for this is as follows. A large amount of Sn can be dissolved within Cu (e.g., bronze), forming a solid solution. Further, Sn significantly decreases the melting point of Cu, lowers the (alpha+liquid phase) dual phase region to the lower temperature side, and concentrates within the liquid phase. In addition, as anticipated form the Hansen""s phase diagram (Alxe2x80x94Sn binary alloys), Sn and Al are dissolved in each other, forming a solid solution only in a liquid phase but they repel each other strongly thermodynamically in both liquid phase and solid phase and do not create intermetallic compounds together. Therefore, part of the liquid phase constituents rich in Sn escapes from the sintered body as the compaction resulting from sintering proceeds. While a large amount of Sn is required for achieving the sinter promoting effect when Sn is added alone, the sweating phenomenon of the Sn-rich liquid phase constituents can be restricted by an addition of Ti so that the liquid phase which promotes sintering comes to exist in the sintered body. As a result, the promotion of sintering by Sn is significantly speeded up.
For restricting the sweating phenomenon, it is preferable to add a small amount of an element which and at least either Sn or Al thermodynamically attract each other. In view of this, Mn, Ni and phosphor iron (Fe-25 wt % P) were checked and verified that they had a function similar to that of Ti. Apart from these elements, the elements (e.g., Fe, Mo, Co, V, and Cr) which can form a noticeable amount of Al compounds and Sn compounds can be found from the Hansen""s phase diagram.
As seen from the Hansen""s phase diagram, the minimum amount of Sn necessary for the involvement of the liquid phase in sintering is 1 wt % or more when sintering temperature is close to 1,000 degrees centigrade and the amount of Sn is preferably limited to 12 wt % or less because the precipitation of brittle intermetallic compounds can be prevented with this.
Since the beta phase, which emerges in Cuxe2x80x94Al binary alloys owing to the addition of Sn, exists at the lower Al concentration side, it is preferable that the sintered material have, as its base, the (alpha+beta) dual phase structure including at least a beta phase, when used for producing a sintered contact component as described later. In this case, the amounts of Al and Sn are adjusted within the range described by the following relational expression. This should be taken into account in cases where the above-described elements (e.g., Ti) which form intermetallic compounds reacting with Al and Sn are added in large amounts.
18.5xe2x89xa62.5xc3x97(Al wt %)+(Sn wt %) 
The preferable amount of Ti is 0.3 wt % or more because Ti is added with intention of restricting the sweating phenomenon as discussed earlier. If the amount of Ti exceeds 10 wt % or more, the amount of liquid phase constituents in sintering becomes excessive as seen from the Hansen""s phase diagram. Accordingly, it is preferable to restrict the amount of Ti to 5 wt % or less for fear that the sweating phenomenon appears and hardening due to the precipitation of intermetallic compounds excessively occurs. This is also economically advantageous.
An addition of P in the form of a pure element powder is difficult and therefore P is generally added in the form of a master alloy powder. However, when adding P in the form of a master alloy powder, noticeable melt-off pores are created during sintering for instance in the case of a phosphor-copper alloy containing 8 wt % P and having a low melting point, which is undesirable for the compaction of the sintered body. In view of this, it is preferable to add P in the form of phosphor-iron alloys (e.g., Fe-25wt % P). The addition of P in the form of phosphor iron is advantageous for the following reasons: the above-described sweating phenomenon and the noticeable melt-off pores generally found in phosphor-copper alloys can be avoided; oxidation due to sintering atmosphere can be considerably restricted; and oxidation coloring (dark brown) of a sintered body can be prevented. These advantages apparently assign an added value to the resulting sintered product and are particularly useful for enhancing bonding when the sintered body is bonded to a metal backing during the sintering process. Further, where the present sintered material is used for a contact component, it is apparently effective to utilize the melt-off pores formed by adding a proper amount of phosphor-copper alloy powder if importance is attached to oil retaining ability. However, if the amount of phosphor-copper alloy exceeds 2 wt % in terms of the amount of P, there will be created excessive melt-off pores, resulting in unfavorable brittleness, and if the amount of phosphor-iron alloy exceeds 2 wt % in terms of the amount of P, the compaction achieved by sintering is disadvantageously impeded.
It is conceivable that an addition of Si enhances sinterability by its function similar to the function of Sn, since Si has substantially the same relationship with Al and Cu as Sn. For example, where Si is added to Cuxe2x80x94Al-1 wt % TiH, the sinter promoting effect of Si is noticeably admitted. However, if the amount of Si exceeds 3 wt %, noticeable hardness and brittleness are both observed. Accordingly, the amount of Si is preferably limited to 3 wt % or less.
It is well known that a combinational addition of Si and Mn can improve wear resistance particularly in a copper-based contact material. A combinational addition of Si and Mn is also preferable in the inventive sintered contact material.
An addition of Ni in combination with Al, Sn, Ti or Si is known to form strong intermetallic compounds, leading to increased hardness in a copper-based contact material. In addition, it is obvious that Ni functions together with Mn, Fe etc. to stabilize the beta phase of Cuxe2x80x94Al alloys, retard the eutectic transformation of beta=alpha+gamma and prevent emergence of the brittle (alpha+gamma) structure, for instance, during cooling subsequent to sintering. It is, therefore, favorable to positively add Ni, but its amount is preferably limited to 10 wt % or less and, more preferably, to 5 wt % or less in view of cost performance.
The function of Ni, which stabilizes the beta phase, reduces the amount of Al, leading to emergence of the beta phase so that sintering is facilitated. It is apparent from the Hansen""s phase diagram that examples of elements which facilitate emergence of the beta phase in Cu alloys include Zn, Be, Ga, In, Sb, Si and Sn.
Further, Co, Be, Cr, Mg, Ag, Ti, Si and others are well known as elements which markedly affect the hardness of copper alloys and their positive use for the inventive sintered material has proved to be favorable.
Moreover, in the inventive sintered contact material, known materials such as W, Mo, tool steel atomized powders, SiC, Si4N3, Pb, graphite, MnS, PbS, TiS and various fluorides can be obviously, positively used as a hard dispersing agent or solid lubricant for the purpose of preventing seizure.
It is also apparent that the addition of the above elements such as Ti, Sn, Mn, Ni, Si, Co, Be, Cr, Fe, Mg, Ag, W, Mo, Pb and P can take the form of alloys or compounds in combination with Cu and other alloy elements.
According to the second invention, there is provided a composite sintered contact component formed by sinter-bonding a contact material excellent in hardness and wear resistance to a metal backing, the contact material being obtained, according to the first invention, by adding various alloy elements to a Cuxe2x80x94Al-based sintered material.
In short, the composite sintered contact component of the second invention is formed by integrally sinter-bonding a Cuxe2x80x94Alxe2x80x94Sn based sintered material to a metal backing, the Cuxe2x80x94Alxe2x80x94Sn based sintered material containing at least 1 to 12 wt % Sn and 2 to 14 wt % Al.
In a preferable form of the second invention, one or more of Ti within the range of 0.3 to 5 wt % and Si within the range of 0.5 to 3 wt % is added. Preferably, the second invention has a structure in which a beta phase is present at least within a sintered structure and intermetallic compounds are dispersed within an (alpha+beta) dual phase, the beta phase and/or the bases of the (alpha+beta) dual phase and beta phase. The second invention preferably contains elements such as Mn, Ni and Fe in an amount of 5 wt % or less, the elements stabilizing the beta phase, retarding the eutectic transformation of beta alpha+gamma and providing hardness. Further, a phosphor-iron alloy powder, which increases reducibility in sintering, is preferably added in an amount of 2 wt % or less in terms of P. Preferably, one or more alloy elements selected from the group consisting of P, Zn, Fe, Ni, Co, Mn, Be, Pb, Mo, W, Mg, and Ag and/or at least one of dispersing elements such as WC, graphite and ceramics is contained.
Sn contained in the Cuxe2x80x94Alxe2x80x94Sn based sintered contact material sweats as described earlier, so that Sn tends to segregate, existing on the contact surface of the resulting component during sliding operation. Accordingly, Sn has good seizure resistance as a contact material. It is preferable to add a small amount of an element (e.g., Ti and Ni) which controls the noticeable sweating of Sn to the Cuxe2x80x94Alxe2x80x94Sn based sintered contact material layer, thereby preventing porosity due to the sweating in the process of sintering. It is also preferable to add a hardening element such as Ti, Si, Mn and Ni thereby to adjust hardness and, in consequence, increase the wear resistance of the resulting contact material.
Further, the Cuxe2x80x94Alxe2x80x94Sn based sintered contact material has at least a beta phase in its structure. The beta phase is a hard phase having a vickers hardness of Hv 200 or more and superior to the alpha phase in terms of adhesion resistance. It is conceivable that when the material is cooled down to room temperature after sintering, most of the beta phase constituents are martensite-transformed into a betaxe2x80x2 phase. However, the shape memory effect achieved by the martensitic transformation from the beta phase (untransformed phase) to the betaxe2x80x2 phase as well as the reverse transformation from the betaxe2x80x2 phase to the beta phase at the time of adhesion to the contact surface would prevent growth of damage caused by adhesion, since the martensitic transformation point (Ms point) is close to room temperature. Since this and the effect of improved tenacity would be expected, the inventive sintered contact material is structured to have the (alpha+beta) dual phase and the beta phase as a base, and intermetallic compounds composed of alloy elements such as Ti, Mn and Ni and alloy elements such as Al, Sn and Si are dispersed in the above structure in proper amounts. The term, xe2x80x9cbeta phasexe2x80x9d appearing in the invention is defined as a beta phase state at sintering temperature. It is well known that, as discussed earlier, most of the constituents of the beta phase are martensite-transformed into the betaxe2x80x2 phase when the sintered material has been cooled down to room temperature after sintering. Therefore, the meaning of the term xe2x80x9cbeta phasexe2x80x9d in the invention includes the state of the betaxe2x80x2 phase.
By virtue of the (alpha+beta) dual structure, the crystal grains of the sintered material become smaller so that uniform extension of the contact surface during adhesion/deformation is enhanced, whereas tenacity and adhesion resistance are increased by restricting abrupt hardening during the processing.
In this case, although it is anticipated that wear resistance decreases, while the removal of wear chip powder from the contact surface is improved, wear resistance can be increased by dispersion of the above-listed proper intermetallic compounds. It is known that the effect of the dispersion of the intermetallic compounds is observed when they are added in an amount of 0.2% by volume or more. Accordingly, in the invention, the lower limit of the precipitating amount of intermetallic compounds is preferably 0.2% by volume or more, whereas the upper limit depends on the application of the material (that is, which will be regarded as important among tenacity, adhesion resistance and wear resistance) and therefore cannot be particularly decided. Where the amount of the intermetallic compounds exceeds 35% by volume, the material often becomes brittle, so that the preferable upper limit is 35% by volume or less. Apparently, the precipitation of the intermetallic compounds in an amount of 0.2% by volume can be accomplished with an addition of about 0.1 wt % or more of the above elements. Therefore, the lower limit of the amount of the element added for the purpose of hardening is preferably controlled in consideration of the above value.
When utilizing the sintered contact material to form a contact component, the powder blend of the contact material is preferably compacted into a specified form and then sintered at a proper temperature for compaction. More preferably, the powder blend is formed into a plate-like shape and once sintered at a temperature of 800 degrees centigrade or more, thereby forming a sheet-like sintered body which is, in turn, mechanically compressed by rolling and then subjected to resintering. By carrying out this process at least once, a compact, hard sintered contact material can be easily produced. After being subjected to bending into a round shape, this sintered contact material is welded or clinched, and then machined into a final shape to form a bushing. The inventive Cuxe2x80x94Alxe2x80x94Sn based sintered material can be much more easily welded than hard high-strength brass-based contact materials, so that an extremely economical manufacturing method can be realized.
According to the invention, the above sheet-like compacted or sintered body is provided with a plurality of independent holes and processed into a round shape. Then, the rounded body is subjected to the same processing as in the above case, thereby forming a cylindrical bushing. These independent holes are utilized as storage holes for supplying various lubricants for lubrication. This process not only makes the oil replenishment intervals longer but also produces a sintered contact material with holes at much lower cost compared to cases where a cylindrical body is holed by machining.
There is known a method for manufacturing a composite sintered contact component in which after a sintered contact material has been sinter-bonded to a metal backing, bending into a round shape is carried out followed by welding or mechanical clinching, and then, the inner and outer faces of the material are machined (in the case of bushings). As discussed earlier, in the invention, the Cuxe2x80x94Alxe2x80x94Sn based sintered material is compressed by promoting sinterability by the addition of the various alloy elements at sintering temperatures of about 900 degrees centigrade or more. By utilizing the noticeable expandability at temperatures lower than the sintering temperature that enables compaction, the second invention is arranged such that: A cylindrical compact formed from the powder blend of the inventive sintered material is placed within the inner circumferential portion of a steel pipe used as a metal backing, the cylindrical compact having an outer diameter as large as or slightly smaller than the inner diameter of the steel pipe. After the cylindrical compact has been bonded to the inner circumferential surface of the metal backing at temperatures lower than the temperature range which provides compaction, the cylindrical compact bonded to the metal backing is compactedly sintered at temperatures of 900 degrees centigrade or more. With this process, a composite sintered contact component, in which the Cuxe2x80x94Alxe2x80x94Sn based sintered material is sinter-bonded to the inner circumferential surface of the metal backing, can be produced and, moreover, this composite sintered contact component can be economically manufactured without the conventionally utilized external pressure exerted from the bore portion.
To sum up, the second invention is designed such that a powder blend containing 2 to 14 wt % Al added in the form of Cuxe2x80x94Al based alloy powder or Al powder is compressed to form a desired cylindrical green compact which is, in turn, inserted into a metal backing having a bore slightly larger than the outer diameter of the green compact. Then, sinter-bonding is performed on the green compact at a temperature of 800 degrees centigrade or more in a sintering furnace controlled to have a vacuum, neutral or reduced atmosphere, whereby a composite sintered contact component in which the sintered material is bonded to the inner circumferential portion of the metal backing is produced.
Preferably, the sinter bonding of the green compact to the inner circumferential portion of the metal backing is carried out by use of a third metal alloy interposed between the metal backing and the green compact. In this case, the third metal alloy may consist of an ingot alloy and/or sintered alloy which create a liquid phase necessary for the bonding to the metal backing at least at the sinter-bonding temperature. The inner circumferential portion of the metal backing may be grooved such that the groove becomes an oil pool for lubricants after the sinter bonding. In addition, the metal backing may be steel.
According to the second invention, in cases where a bushing, which requires wear resistance and adhesion resistance as critical factors, is formed from the Cuxe2x80x94Alxe2x80x94Sn-based sintered material having a structure wherein an (alpha+beta) dual phase containing many hard beta constituents or a beta phase is created as a base and intermetallic compounds are dispersed, no cracking is caused by the above-described round shape bending in the sintered contact material.
As an alternative method for the above-discussed sinter bonding of the cylindrical green compact formed from the powder blend, the cylindrical composite sintered contact component can be manufactured by the following way: A sheet-like compact made from a powder blend is preliminarily sintered, rounded, and then sinter-bonded, being placed within the inner circumferential portion of the steel pipe.
At the time of compacting or after preliminary sintering, the sheet-like compact is provided with a plurality of independent holes which are utilized as storage holes for various lubricants so that lubrication is provided for the contact surface of the resulting cylindrical composite sintered contact component. Thanks to this arrangement, the composite sintered contact component has longer lubricant replenishment intervals.
As described above, a compact, hard sintered contact material can be produced by carrying out a process at least once, in which a sheet-like sintered body achieved by once sintering at 800 degrees centigrade or more is subjected to mechanical compaction by rolling and, then, subjected to resintering. This sintered contact material may be bent into a round shape, and then welded or clinched (i.e., geometrical bonded). With this process, the material can be easily shaped into e.g., a bushing. This process apparently presents the following advantages: (i) Materials (e.g., intermetallic compounds), which are poor in reactivity with respect to e.g., Cu and cannot be used in the form of an ingot, can be dispersed in the form of minute grains having sizes of 1 xcexcm or less; and (ii) W, Mo, ceramics, tool steel powder, WC, hard metals, cement, solid lubricants and others can be dispersed.
The inventors have developed an integral, composite, sintered contact component by sinter-bonding a Cuxe2x80x94Alxe2x80x94Sn-based sintered contact material to a steel plate, based on such findings that excellent contact properties (e.g., tenacity during sliding movement, seizure resistance and wear resistance) equivalent to or superior to those of Pb-bronze-based sintered contact material can be achieved by the above described structure having the fine (alpha+beta) dual phase as a base. The inventive composite sintered contact component is particularly expected to play an increasingly important role in coping with the recent environmental problems as a Pb-free sintered contact component.
If the amount of elements such as Ti, Si, Ni, Mn and FeP which form compounds is increased, the resulting contact material becomes more likely to attack its mating member when it moves in sliding contact with the latter, and therefore, it is desirable to reduce the above elements as much as possible in the (alpha+beta) dual phase structure of the Cuxe2x80x94Alxe2x80x94Sn based composite sintered contact component.
The Cuxe2x80x94Alxe2x80x94Sn based composite sintered contact component sinter-bonded to the steel plate may be formed such that after the powder blend has been compressed (e.g., by rolling), the compressed body is sintered at least twice at 700 degrees centigrade or more in a furnace controlled to have a vacuum, neutral, or reduced atmosphere, whereby the difficulty of sintering the Cuxe2x80x94Al based sintered material due to the formation of an oxidized film is overcome by oxidized film destruction caused by compression molding, so that a highly tough sintered material can be obtained even by low-temperature sintering, which sintered material is a Pb-free contact material hardly attacking its mating material and having the fine (alpha+beta) dual phase structure as a base.
More concretely, the powder blend of Cuxe2x80x94Alxe2x80x94Sn based sintered contact material composed of at least a bronze powder, copper powder, Sn powder, Al powder and TiH powder is sprayed onto the steel plate, and sinter-bonding is carried out at 700 degrees centigrade or more in a sintering furnace controlled to have a vacuum, neutral or reduced atmosphere. Then, compression molding (e.g., rolling) and the above-described sintering process at 700 degrees centigrade in the same sintering atmosphere are preferably repeated twice or more, thereby obtaining a Cuxe2x80x94Alxe2x80x94Sn composite sintered contact component. It is desirable to use an atomized powder particularly in view of scatterablility. For achieving more uniform bonding, a bronze atomized powder needs to be used properly. In addition, the amount of Sn contained in the Cuxe2x80x94Alxe2x80x94Sn based sintered contact material is preferably 3 wt % or more in order to ensure good bonding with respect to the steel plate, whereas the amount of Al is controlled in consideration of the quantitative relationship between the alpha phase and the beta phase. For example, in the case of Cuxe2x80x94Alxe2x80x94Snxe2x80x94Ti ternary alloys containing 3 wt % Sn and 1 wt % Ti, the (alpha+beta) dual phase is created where the amount of Al is 10 wt %, but where the amount of Al is about 12 wt % or more, the material has a single beta phase and therefore excessive hardness, resulting in poor tenacity. Therefore, the amount of Al should not exceed about 12 wt %.
It is desirable to lessen the amount of Al while increasing the amount of Sn in order to ensure stable bonding with respect to the steel plate, but the amount of Sn needs to be reduced in view of the cost performance of the Cuxe2x80x94Alxe2x80x94Sn based sintered contact material.
Taking the above into account, the invention is characterized by the following process: A bronze-based, Cuxe2x80x94Sn alloy powder or powder blend containing 5 to 12 wt % Sn is sprayed onto a steel plate; and then the alloy powder or powder blend is sinter-bonded to the steel plate at 700 degrees centigrade or more in a sintering furnace controlled to have a vacuum, neutral or reduced atmosphere to form a metal backing. The above-described Cuxe2x80x94Al and/or Cuxe2x80x94Alxe2x80x94Sn based alloy powder having an Al concentration of 2 to 14 wt % is sprayed onto the metal backing. Then, the steps of (i) sintering and rolling at 700 degrees centigrade or more; (ii) sintering or rolling at 700 degrees centigrade or more; and (iii) sintering at 700 degrees centigrade or more are carried out thereby obtaining the composite sintered contact component.
Herein, it is preferable to further repeat, twice or more, the steps of (i) sintering and rolling at 700 degrees centigrade or more; (ii) sintering or rolling at 700 degrees centigrade or more; and (iii) sintering at 700 degrees centigrade or more, whereby the Cuxe2x80x94Al and/or Cuxe2x80x94Alxe2x80x94Sn based sintered contact layer is fined so as to have an average grain size of 5 xcexcor less and, at the same time, compacted so as to have a relative density of 90% or more.
In addition, the fining of the crystal grains of the Cuxe2x80x94Alxe2x80x94Sn based sintered material can be accomplished by: (i) restraining the growth of the grains owing to the dual phase structure during sintering; (ii) sintering at low temperatures carried out by the above-described repetition of compression/sintering and fining by recrystallization; and (iii) the addition of the elements (e.g., Ti, Si, Ni) which are likely to form compounds. It should be noted that when sintering temperature is 700 degrees centigrade or less, alloying reaction becomes slow and sufficient deformation cannot be ensured in the compression process, resulting in cracks within the sintered body, even though a liquid phase is generated. Therefore, desirable sintering temperature is 800 degrees centigrade or more.